1. Field of the Invention
This invention relates to photodetectors, and particularly to photodetectors that have charge carrier reflectors.
2. Description of the Related Art
Desirable characteristics of commercial photodetectors include high quantum efficiency (QE), otherwise known as sensitivity, increased signal-to-noise ratio and the ability to manufacture them with reduced manufacturing defects. A photodetector with higher QE may translate into faster image capture, while increased signal-to-noise ratio means fewer image artifacts when used with reduced illumination. Reducing manufacturing defects translates into increase wafer yields, higher detector array operability and reduction in the unit cost for each photodetector.
FIG. 1 illustrates a prior art single-pixel photodetector 100. The photodetector includes an absorber layer 105 that produces electron-hole pairs (charge-carriers) in response to absorbed photons, a buffer layer 110 to inhibit such charge-carriers from drifting into the substrate 115, and a collector layer 120 that forms a P—N junction with the absorber layer 105 to collect the minority charge carriers (holes). Majority carriers (electrons) are collected by the absorber layer contact 130. Because charge carriers typically recombine after a predetermined average diffusion length, lateral spreading of the charge carriers results in reduced QE as they recombine prior to capture by the collector layer 120. The absorber layer itself may produce spurious charge carriers from thermal effects, rather than photon absorption, to generate dark current between a collector layer contact 125 and absorber layer contact 130 thereby reducing the signal-to-noise ratio. Also, any significant defects at the P—N junction between the junction and absorber layers 120, 105 can result in a defective photodetector and reduced wafer yields for the photodetectors. While reducing the area of the collector layer 120 would reduce the opportunity to develop manufacturing defects at the P—N junction and would increase wafer yields, such a resizing would reduce QE due to charge carrier drift and recombination prior to capture by the collector layer 120, since fewer of the charge carriers would be likely to reach the reduced collector layer 120. Resizing also would not be as effective in reducing dark current which limits the signal-to-noise ratio for the photodetector.
A need still exists, therefore, to increase charge carrier capture prior to recombination, to reduce generation of thermally induced charge carriers and to increase photodetector manufacturing yields without adversely affecting overall QE.